1. Field of the Invention
The present invention relates to a composite material of an organic compound and an inorganic compound, which has an excellent carrier transport property and carrier injection property into the organic compound and also has excellent visible light transmittance. The invention also relates to a current-excitation light emitting element using the composite material. The invention further relates to a light emitting device having the light emitting element.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed. A basic structure of this light emitting element is that a layer containing a light emitting organic compound is sandwiched between a pair of electrodes. By applying a voltage to this element, electrons and holes are separately injected from the pair of electrodes into the layer containing a light emitting organic compound, and current flows. Then, recombination of these carriers (the electrons and holes) causes the light emitting organic compound to form an excited state and to emit light when the excited state returns to a ground state. Owing to such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.
Note that an excited state of an organic compound can be a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight, since the light emitting element is formed of an organic thin film, for example, with a thickness of approximately 0.1 μm. In addition, extremely high response speed is another advantage, since time between carrier injection and light emission is approximately 1 μsec or less. These characteristics are considered suitable for a flat panel display element.
Such a light emitting element is formed in a film shape. Thus, surface emission can be easily obtained by forming a large-area element. This characteristic is hard to be obtained by a point light source typified by an incandescent lamp or an LED or a line light source typified by a fluorescent lamp. Therefore, the above described light emitting element also has a high utility value as a surface light source which is applicable to lighting or the like.
As described above, the current-excitation light emitting element using the light emitting organic compound is expected to be applied to a light emitting device, lighting, and the like. However, there are still many issues. One of the issues is a reduction in power consumption. It is important to reduce a drive voltage of the light emitting element in order to reduce power consumption. Since emission intensity of the current-excitation light emitting element depends on the amount of current flowing therethrough, it is necessary to make a large amount of current to flow at low voltage in order to reduce the drive voltage.
It has been attempted so far to provide a buffer layer between an electrode and a layer containing a light emitting organic compound, as a technique for reducing a drive voltage. For example, it is known that a drive voltage can be reduced by providing a buffer layer which is formed of polyaniline (PANI) doped with camphorsulfonic acid between indium tin oxide (ITO) and a light emitting layer (for example, Reference 1: Y. Yang et. al., Applied Physics Letters, Vol. 64 (10), 1245-1247 (1994)). It is explained that this is because PANI has a superior carrier injection property into the light emitting layer. Note that PANI which is the buffer layer is considered as part of the electrode in Reference 1.
However, as stated in Reference 1, PANI has a problem in that transmittance thereof becomes lower as a thickness thereof increases. Specifically, it is reported that a transmittance is below 70% when a thickness is approximately 250 nm. In other words, there is a problem with transparency of a material itself which is used for the buffer layer; therefore, light generated inside an element cannot be extracted efficiently.
According to Reference 2, it is attempted to improve a luminance per certain current density, in other words, current efficiency by serially connecting light emitting elements (referred to as light-emitting units in Reference 2) (Reference 2: Japanese Patent Laid-Open No. 2003-272860). In Reference 2, a mixed layer of an organic compound and metal oxide (specifically, vanadium oxide and rhenium oxide) is used for a connecting portion at the time of serially connecting the light emitting elements, and it is assumed that this layer can inject holes and electrons into the light-emitting unit.
However, as can be understood from the embodiment, the mixed layer of an organic compound and metal oxide, which is disclosed in Reference 2, has a strong absorption peak in a visible light region (in the vicinity of 500 nm) as well as an infrared region, and also has a problem with transparency. Consequently, after all, light generated inside the element cannot be extracted efficiently, and luminous efficiency of the element is lowered.